Electrospray ionization (“ESI”) has revolutionized the use of mass spectrometry in bioanalytical chemistry because of its ability to transfer large macromolecules from solution into the gas-phase as intact multiply-charged molecular ions. A special advantage of ESI is the ease with which it may be coupled to liquid chromatography (“LC”), capillary electrophoresis (“CE”), and capillary electrochromatography (“CEC”). An attractive development in recent years has been the design of methods for decreased sample consumption in ESI by using much lower flow rates (nL/min) than with conventional ESI (μL/min). Of these low flow ESI methods, the flow rate is controlled by some type of pump in microspray and flow-through nanospray, whereas in static nanospray the flow rate is controlled by the potential difference between the emitter and counter-electrode, and some gas pressure may also be applied to maintain the spray, where flow from the tip is typically 30-60 nL/min. Typically, nanospray emitters have been fabricated by pulling silica or glass substrates under heat to produce tapered emitters with small inner diameters, e.g., a few μm.
For nanospray ESI-MS emitters to be useful in coupling to either LC, CE or CEC, the emitters must remain stable throughout the separation process. Failure of the emitter during the course of the separation is not acceptable. For quantification in particular, if calibration curves of multiple analytes at multiple concentration levels are to be constructed, single emitters with longer lifetimes or multiple emitters showing reproducible performance and ionization efficiency are needed. The stationary bed must remain secure throughout the lifetime of the column apparatus.
While nanospray provides an avenue to achieve low-level detection limits with MS using only a few μL of sample, even at high salt and/or buffer concentrations, most nanoliter-flow ESI emitters suffer from short operating lifetimes, poor durability, and/or low reproducibility. Additionally, if the internal diameter of the emitter is too large, or there is too much dead volume associated with coupling the emitter to the outlet of the column, then band broadening can often be a problem thereby compromising effective analysis.
Presently, there is a need for an emitter that can overcome the deficiencies in the art as currently practiced.